Known means of determining a quality level of an analog transmission link include, for example, a technique of measuring an average signal strength of an intermediate frequency stage in a radio receiver to establish an audio squelch signal. The technique uses a measurement of a radio frequency (RF) carrier to infer the quality of the analog signal during the use of the link. The technique of deriving a squelch signal, while being simple and widely used, suffers from some deficiencies, amongst which is that the squelch signal can indicate a good audio signal when the audio signal is distorted but while the RF carrier is strong, for example, when the desired signal is interfered with by an adjacent channel RF signal. In a related example, a test signal used in a setup of a telephone modem link determines a quality level of the telephone link at the setup time of the link. Some techniques use a predetermined audio signal. This modem setup technique determines a quality level for the link at the time of setup, but not generally thereafter, and is normally for the use of a analog telephone link for the transmission of data. The quality of the telephone link after setup is generally determined by digital methods such as bit error rate.
Neither of these techniques solve the problem of accurately measuring the quality of an analog signal while it is being transmitted over a radio link, and in particular, are not readily adaptable to accurately measuring the quality of a voice signal over a radio link. Being able to accurately measure the quality of an analog signal transmitted over a radio link is important when the analog signal is transmitted as a non-real-time signal in a system having a protocol allowing acknowledgment of the analog signal, such as a paging system sending voice messages that can be acknowledged or which, when not acknowledged, can be retransmitted. In such a system, the average quality of analog messages delivered to radio receivers can be improved. Another example in which being able to accurately measure the quality of an analog signal transmitted over a radio link is important is when the analog signal is a voice signal which is transduced on a real-time basis and transmitted in an radio environment which includes high levels of interference. In such a system, a high accuracy quality signal can provide a better squelch function.
Thus, what is needed is an improved technique for accurately measuring the quality of an analog signal in radio communication system.